Fanning Deep Injection of Botulinum Toxin Type-A for Masseter Hypertrophy: A Retrospective Study of 239 Patients

Masseter hypertrophy commonly contributes to a broadened lower-face contour in Asian populations. Although botulinum toxin type A (BoNT-A) is widely used for nonsurgical facial contouring, conventional multi-point injection techniques may result in uneven diffusion, paradoxical bulging, cheek hollowing, or smile asymmetry. The Fanning Deep Injection (FDI) technique was developed as an anatomically guided approach targeting the deep-inferior masseter through a single-entry, fan-shaped distribution pattern.

We retrospectively reviewed patients treated with BoNT-A using the FDI technique between January 2021 and December 2023. Eligible patients had standardized pre-/post-treatment photographs (≥3 months), complete procedural records, and available follow-up data. Outcomes included injection pain (VAS), facial-width changes measured on standardized frontal photographs, patient satisfaction (GAIS and Likert scale), and treatment-related complications.

Among 6,520 treated patients, 239 met all inclusion criteria. The mean age was 30.3 ± 6.3 years, and the mean follow-up duration was 4.86 ± 1.28 months. Significant reductions in facial width were observed across all three horizontal reference lines, with the greatest reduction at the oral-commissure level. Injection-related discomfort was minimal (VAS 1.02 ± 0.82), and patient satisfaction was high. Two patients (0.8%) experienced transient localized bulging that resolved spontaneously. No clinically significant cheek depression, persistent smile asymmetry, or long-term masticatory weakness requiring intervention was identified during the available follow-up period.

Within the limitations of this retrospective study, the FDI technique demonstrated favorable aesthetic outcomes, high patient satisfaction, and a low incidence of adverse events in the treatment of masseter hypertrophy. The deep-inferior fan-shaped injection strategy may facilitate relatively homogeneous intramuscular distribution while potentially reducing superficial diffusion to adjacent mimetic muscles. However, prospective controlled studies incorporating longer follow-up, standardized ultrasound protocols, validated patient-reported outcome measures, and 3D quantitative imaging are required to further validate these findings.

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